Uncertainty of eddy covariance flux measurements over an urban area based on two towers

• Published in: Atmospheric measurement techniques 2018-10, Vol.11 (10), p.5421-5438
• Main Authors: Järvi, Leena, Rannik, Üllar, Kokkonen, Tom V, Kurppa, Mona, Karppinen, Ari, Kouznetsov, Rostislav D, Rantala, Pekka, Vesala, Timo, Wood, Curtis R
• Format: Article
• Language: English
• Subjects: Aerodynamics; Air pollution; Area; Buildings; Carbon; Carbon dioxide; Carbon dioxide flux; Complications; Covariance; Covariance analysis; Data; Data analysis; Data processing; Ecological monitoring; Ecosystems; Eddies (Fluid dynamics); Eddy covariance; Environmental aspects; Filtration; Flow distortion; Fluctuations; Fluid dynamics; Fluxes; Greenhouse effect; Greenhouse gases; Heat exchange; Heat transfer; Kinetic energy; Kurtosis; Latent heat; Measurement; Methods; Metropolitan areas; Momentum; Momentum flux; Momentum transfer; Pollutants; Pollution monitoring; Roughness; Sensible and latent heat; Sensitivity; Skewness; Turbulent heat transfer; Turbulent kinetic energy; Uncertainty; Urban areas; Urban environments; Vortices; Wind
• ISSN: 1867-8548; 1867-1381; 1867-8548
• DOI: 10.5194/amt-11-5421-2018

The eddy covariance (EC) technique is the most direct method for measuring the exchange between the surface and the atmosphere in different ecosystems. Thus, it is commonly used to get information on air pollutant and greenhouse gas emissions, and on turbulent heat transfer. Typically an ecosystem is monitored by only one single EC measurement station at a time, making the ecosystem-level flux values subject to random and systematic uncertainties. Furthermore, in urban ecosystems we often have no choice but to conduct the single-point measurements in non-ideal locations such as close to buildings and/or in the roughness sublayer, bringing further complications to data analysis and flux estimations. In order to tackle the question of how representative a single EC measurement point in an urban area can be, two identical EC systems - measuring momentum, sensible and latent heat, and carbon dioxide fluxes - were installed on each side of the same building structure in central Helsinki, Finland, during July 2013-September 2015. The main interests were to understand the sensitivity of the vertical fluxes on the single measurement point and to estimate the systematic uncertainty in annual cumulative values due to missing data if certain, relatively wide, flow-distorted wind sectors are disregarded.